Carbon fibers used as reinforcing fibers and so forth of fiber-reinforced composite are fibers in which no less than 90 weight % of the chemical composition is composed of carbon, and which are produced by using regenerated cellulose, polyacrylonitrile (PAN) or pitch and so forth as starting material. These carbon fibers are divided into, for example, high-strength carbon fibers, high-modulus carbon fibers and so forth.
Since carbon fibers are lightweight, have particularly superior properties with respect to specific strength and specific modulus and also have superior heat resistance and chemical resistance, they are particularly effective as reinforcing fibers of fiber-reinforced composite, and are used over a wide range of applications.
In addition, resin compounds such as epoxy resin or inorganic compounds such as cement and ceramics are used for the matrix of fiber-reinforced composite using carbon fibers as reinforcing fibers, and fiber-reinforced composite are formed that have superior mechanical properties.
In recent years, carbon fibers have also come to be used as reinforcing fibers of short-fiber reinforced composite materials. For example, after chopping carbon fibers into the form of chopped carbon fibers, they are dispersed in water to produce paper containing randomly dispersed chopped carbon fibers and so forth. In addition, chopped carbon fibers are also uniformly mixed and stirred into an inorganic matrix slurry such as concrete to produce a short-fiber reinforced composite in which chopped carbon fibers are randomly dispersed.
In order to obtain chopped carbon fibers for use in these applications, carbon fibers are typically treated with a water-soluble sizing agent followed by chopping. Preferable sizing agents used for this purpose are provided with superior solubility in water, while also being able to impart both the necessary convergence required for forming stable chopped carbon fibers as well as superior uniform tow dispersibility in water to the carbon fibers.
In addition, there are also many applications in which fabrics using carbon fibers are suspended in water or immersed in an aqueous matrix to impregnate with that matrix. Carbon fibers used in these applications are required to have superior workability (e.g., processability) when in the form of a fabric and superior uniform tow dispersibility in water. Treatment using a water-soluble sizing agent is also carried out to obtain carbon fibers provided with these properties.
Here, examples of known sizing agents include sizing agents composed of bisphenol type polyalkylene ether epoxy compounds (Japanese Unexamined Patent Application, First Publication No. Sho 61-28074), sizing agents composed of compounds in which several ten molecules of alkylene oxide are added to bisphenol A (Japanese Unexamined Patent Application, First Publication No. Hei 1-272867, Japanese Unexamined Patent Application, First Publication No. 7-9444), polyvinyl alcohol and water-soluble thermoplastic resins such as water-soluble Nylon resin (Japanese Examined Patent Application, Second Publication No. Hei 5-4348 or Japanese Patent Publication No. 2838309).
However, conventional sizing agents have the problems described below.
The sizing agents composed of bisphenol type polyalkylene ether epoxy compounds indicated in Japanese Unexamined Patent Application, First Publication No. Sho 61-28074 have comparatively satisfactory converging performance and allow the obtaining of carbon fibers having superior processability and other workability when formed into chopped carbon fibers or fabrics as a result of providing a glycidyl group in the compound serving as said sizing agent. However, these sizing agents have the disadvantages of being sticky due to the glycidyl group present in the compound, having inadequate solubility in water, and preventing the obtaining of carbon fibers having satisfactory uniform tow dispersibility in water.
On the other hand, sizing agents composed of a compound in which several ten molecules of alkylene oxide are added to bisphenol A indicated in Japanese Unexamined Patent Application, First Publication No. Hei 1-272867 and Japanese Unexamined Patent Application, First Publication No. Hei 7-9444 is able to impart superior uniform tow dispersibility in water to carbon fibers as a result of having superior solubility in water.
However, since this type of sizing agent composed of a compound containing ethylene oxide has somewhat weak converging performance that can be imparted to carbon fibers, in order to obtain carbon fibers having adequate convergence required for forming stable chopped carbon fibers and superior processability and other workability when forming into fabrics, it has the disadvantage of requiring a large amount of sizing agent to be adhered to the carbon fibers. In addition, the stickiness of this type of sizing agent increases as a result of adsorbing moisture in the air due to the presence of hydrophilic groups such as (—CH2—CH2—O—) in its molecules thereby resulting in sticking which lowers the processability and other workability of the carbon fibers when formed into fabrics. Consequently, in the case of using this type of sizing agent, the amount of sizing agent adhered must be strictly controlled in order to provide satisfactory processability and other workability to the resulting carbon fiber bundles, thereby increasing the complexity of the production process.
Moreover, sizing agents composed of polyvinyl alcohol or a water-soluble thermoplastic resin such as water-soluble Nylon resin indicated Japanese Examined Patent Application, Second Publication No. Hei 5-4348 or Japanese Patent Publication No. 2838309 have superior solubility in water as well as satisfactory converging performance. Consequently, they are able to impart adequate convergence required for forming into stable chopped carbon fibers as well as superior processability and other workability when forming into fabric to the carbon fibers. However, although carbon fibers imparted with polyvinyl alcohol have superior uniform tow dispersibility in an aqueous solution in the vicinity of pH 7, uniform tow dispersibility in acidic or alkaline aqueous solutions is inadequate. In addition, although carbon fibers imparted with water-soluble Nylon resin have superior uniform tow dispersibility in acidic aqueous solutions, their uniform tow dispersibility in neutral or alkaline aqueous solutions is inadequate.
As has been explained above, there are no conventional sizing agents having satisfactory solubility in water over a wide pH range, or which are able to simultaneously impart adequate convergence required for forming stable chopped carbon fibers, superior workability (e.g., processability) to carbon fiber when forming fabrics, and superior uniform tow dispersibility in water over a wide pH range.